1. Field of the Invention
The present invention relates to a semiconductor storage device and, more particularly, to a static random access memory.
2. Description of the Prior Art
A static random access memory (hereinafter referred to “SRAM”) is useful because it does not require refresh operation. However, the number of elements constituting one memory cell in the SRAM is large to increase an occupied area by the memory cell. For this reason, it is required to reduce a cell area. For example, Japanese Laid-open Patent Publication No. 9-27.0468 (U.S. Pat. No. 5,744,844) and Japanese Laid-open Patent Publication No. 10-178110 (U.S. Pat. No. 5,930,163) illustrate examples of cell layout in which one cell having a length in a word line direction which is larger than a length in a bit line direction. Of the examples, a flat configuration of the SRAM described in Japanese Laid-open Patent Publication No. 10-178110 is shown in FIGS. 16 and 17. FIG. 16 is a plan view related to one memory cell of the SRAM. FIG. 17 is an equivalent circuit diagram corresponding to the memory cell shown in FIG. 16. The length in the bit line direction is shortened to increase the speed, and the layout of an active layer and a gate wiring element has a simple shape to reduce a cell area.
From a viewpoint of decreasing patterning size, a phenomenon (optical proximity) in which a resist pattern on a wafer is distorted becomes conspicuous due to the interference of light in an exposure device. In addition, even in an etching process, pattern distortion is generated by the micro-loading effect after etching. The micro-loading effect is a phenomenon in which an etching rate decreases in a direction of depth when a pattern having a large difference in density. In recent years, in order to minimize these pattern distortions phenomena, a technique for the optical proximity correction (OPC) is developed and used in which a mask pattern is automatically corrected in advance in a photography process.
In general, in order to form a contact by forming a contact hole in a gate wiring element, a cover margin such as a lithographic margin and a machining margin must be set in consideration of blur in photolithography process. For this reason, a portion in which a contact hole should be formed on the gate wiring element must be deformed by increasing the width of the portion by a length corresponding to a cover margin. In addition, since a width must be partially increased to make the width of the gate wiring element fine, decreasing patterning size cannot be achieved easily.
In order to decrease patterning size in consideration of an optical proximity correction (OPC) pattern obtained by the optical proximity correction (OPC) technique, when gate wiring elements are complicatedly arranged, margins for the optical proximity correction must be set in the longitudinal and lateral directions. For this reason, a memory cell area cannot be reduced sufficiently because sufficient decrease in size cannot be achieved, and the margins are factors which hinder decrease in size.